Hold on, now: Am I to understand that games downloaded via Galaxy are "Galaxy installations", similar to the way Steam games are tied to the Steam client, and that using Galaxy to acquire an earlier version of an "offline installer" is thus impossible?

Im honestly not sure if you're being ironic. Pardon me if you were and I couldn't grasp the inherent joke present in your post. :P

In any way, explaining this might help Galaxy users only reading this thread to understand our unsatisfaction with how GOG is handling offline installers currently:

We currently have two official ways of downloading our bought products here:
- Using a browser to access the "Games" tab and downloading them directly
- Using Galaxy to install them directly

When downloading games through the browser, you will download a packaged setup containing your game.
This package is created using InnoSetup, a tool that can be used to create setups easily using scripts to automate the process of future installation. What GOG does is package the game files using it with compression methods through scripting using this tool. Said 'Offline Installers' are also divided into packs of 4gb for those of us still using old drives for easy of management and those of us with slow internets for easy of download continuation if something happens during download and they also contain the necessary dependencies for each game to work like Directx, Redist, .NET, PhyX, and so on.

When the download is complete, you can then install it using the main installation executable, which then creates the necessary registry entries, install the dependencies... or extract the installers using tools like InnoExtract, which just do the process of InnoSetup on reverse, basically, without creating the Registry Entries or installing the dependencies, if you are a more advanced user and want only the game files for backup.

Galaxy installations are different. In that sense, yes, they're just like Steam. Galaxy connects to GOG servers and download file by file directly to the defined installation folder, creating the registry entries later and installing the dependencies on top of it.

When we ask for Offline Installers Rollback in this thread, I think we collectively mean we want a way to download the Offline Installers Pre-Preservation Program builds that we had access before they were force updated to the Preservation Program builds.

Why Galaxy users can, indeed, rollback their installation to a previous build while Offline Installers users can't?
That's of course not the only problem, but then the forum is full of threads about all the disparities between them and they're easy to find.

If anyone finds any information wrong in this post, please, feel free to correct me. I'd be thankful.

That's how it worked the last time I looked. "Rolling back" via Galaxy will patch an already installed game to an older version, but unless something has changed recently, it still doesn't actually give you an older offline installer to permanently archive / backup. So if GOG were to go "kaput", even Galaxy users won't be able to "rollback" to that older version at a later date if there's no Galaxy server to connect to (or reinstall / play the game at all if they didn't back anything up).

You forgot to mention that those same offline installers can also be downloaded with the client for later use and with less clicks whenever an installer is split to multiple parts.

Apologies OP, wasn't trying to get the thread closed. That said, I think the thread has run it's course and everything has been said.